Americans seeking to assist the people of Gaza can legally do so through highly respected non-governmental organizations such as CARE and Oxfam.
In fact, the United States Agency for International Development gives grants to the following additional organizations for Gaza aid operations: American Near East Refugee Aid (ANERA), Catholic Relief Services (CRS), CHF International, International Orthodox Christian Charities (IOCC), Mercy Corps, Relief International, Save the Children, World Vision, and World Food Program. All of these organizations accept private donations.
The Israeli government and the United Nations operate aid corridors at land-border crossings. These allow the Israeli government to ensure that no weapons are brought into the territory and that the aid makes its way to groups that will provide it to the Gaza people, not to Hamas or other terrorists.
It is a crime to provide assistance of any kind to Hamas, which is a foreign terrorist organization. However one feels about the blockade and embargo or the wider politics of the Arab-Israeli dispute, people subject to United States (U.S.) jurisdiction cannot provide assistance to groups designated by the U.S. government as terrorist organizations. By bypassing the legitimate aid corridors, the participants in the flotilla knew that the material they carried would be given to Hamas once the ships reached Gaza's port.
Several of the flotilla participants have contacts with the Hamas organization. Photos from an early Free Gaza visit to the region shows members of the group, including a U.S. participant in the May 31 flotilla, meeting with Hamas leader Ismail Haniyeh and receiving medals from the group.
One of the groups responsible for organizing the flotilla, the Insani Yarhim Vakfi (IHH) was accused in testimony provided by a highly respected French counter-terrorism jurist of playing "an important role" in planning the Al Qaeda plot to bomb the Los Angeles airport in 2000. The umbrella organization to which IHH belongs, the Union of Good, was designated under Executive Order 13224 in 2008 by the Treasury Department -- specifically for being a fundraiser for Hamas.